1. SCAMP-Like Polypeptides
Considerable evidence for endocytosis in plants has accumulated during recent years (Samaj et al., 2004; Plant Physiol. 135: 1150-1161). Some of the components of the clathrin-based internalization machinery have been identified and data for the uptake of cell surface receptor-ligand complexes is accumulating (Russinova et al. 2004, Plant Cell 16: 3216-3229). Recently it has been hypothesized that plant SCAMP proteins might play a role in mediating endocytosis in plant cells (Lam et al. 2007; The Plant Cell, Vol. 19: 296-319). SCAMP proteins were Initially identified as secretory vesicle components in mammalian exocrine glands and later found to be ubiquitous proteins in eukaryotes (Fernandez-Chacon and Sudhof, 2000; J. Neurosci. 20: 7941-7950). SCAMPs were found in both the trans-Golgi and the endosomal recycling compartment, and they appear to be concentrated within the motile population of early and recycling endosomes (Castle and Castle, 2005) J. Cell Sci. 118: 3769-3780. Plant SCAMP homologs have been found amongst others in rice (Oryza sativa), Arabidopsis, and pea (Pisum sativum) and are thought to be present in many other plant species (Fernandez-Chacon and Sudhof, 2000). In plants SCAMPs have been localized at the plasma memebrane and the and mobile cytosolic organelles (Lam et al. 2007).
2. Fibrillin Polypeptides
The most prominent proteins in plastoglobulins (PGs) are fibrillins. Fibrillins are plastid-associated lipid-binding proteins that are ubiquitous in plants and cyanobacteria. They have been primarily characterized from chromoplasts of tomato and pepper fruits and are known to accumulate during abiotic stress in plastids e.g., inflicted by high light, cold, and drought, and also during pathogen infection. The family of fibrillin-like proteins contain a hydrophobic domain that associates with or anchors within lipids. Fibrillins associate with stromal lamellae of thylakoids and fibrillic carotenoid-containing structures of chromoplasts. A model for the fibrillic structures predicts a layer of fibrillin shielding polar lipids and carotenoids. Furthermore, fibrillin is known to accumulate during high-light conditions, and fibrillin affects photosynthetic efficiency (see Yang et al., Proc Natl Acad Sci USA. 2006 April 11; 103(15): 6061-6066). Evidence is also available for the association of these proteins to various lipid globules under non-stressed conditions preventing plastoglobule coalescence (see CAB Abstracts, Simkin et al., Recent Research Developments in Biochemistry, 2004).
The Arabidopsis genome has 13 fibrillin genes that are all predicted to encode plastid localized proteins (Laizet et al., 2004). Rey et al., (Plant J. 2000 March; 21(5):483-94) disclose transgenic Nicotiana tabacum plants over-expressing fibrillin using a constitutive promoter. No growth difference between wild-type plants and transgenic plants was noticed under low light conditions, however transgenic plants were reported to exhibit a longer main stem, enhanced development of lateral stems and accelerated floral development under higher light intensities.
3. PLATZ Polypeptides
PLATZ proteins form a plant specific family of DNA-binding proteins. So far, only one member has been described in more detail (PLATZ1, Nagano et al, Nucl. Acids Res. 29, 4097-4105, 2001). Sequence comparison between PLATZ1 and other putative PLATZ proteins revealed the presence of two Zn-binding domains with conserved cysteine and histidine residues. DNA-binding activity required the presence of Zn. PLATZ1 was shown to bind A/T-rich regions in a non-specific way, and was able to induce expression of the GTPase pra2 and plastocyanin petE genes (Nagano et al., 2001). Though DNA-binding proteins are implicated in DNA replication and in regulation of gene expression, a precise characterisation of the role of PLATZ proteins is still lacking.
4. Glomalin Polypeptides
Glomalin was first identified as a high molecular mass glycoprotein produced by the arbuscular mycorrhizal fungi (like Glomus sp). It is secreted into the environment and the sugar moiety was postulated to play a role in sequestering of Cu and Zn in the soil. Gadkar and Rillig (FEMS Microbiol Lett. 263, 93-101, 2006) have shown that the glomalin of Glomus intraradices is a protein of 590 amino acids with three N-terminal glycosylation sites and a string of GGM motifs at the C-terminal end. The genomic sequence had three introns of 67, 76 and 131 bp length. The protein had homology to heat shock protein 60 (hsp 60); a plant homologue of hsp60 reportedly plays a role in acclimating photosynthesis to heat stress, possibly by protecting Rubisco activase from thermal denaturation (Salvucci M., E., J Exp Bot. 2008; 59(7):1923-33). However, the precise role of glomalin orthologues in plant biology remains to be elucidated.